1. Field of the Invention
The present invention provides a selective fluorescent chemosensor, sensitive to nanomolar concentrations of zinc(II), Zn.sup.2+ , a device containing the same, and methods for measuring the concentration of a metal in a sample using the device.
2. Discussion of the Background
The selective and quantitative detection of trace amounts of divalent zinc (Zn.sup.2+ ) is commercially desirable for the diagnosis of metal ion induced disease states and in the protection of the environment. Zinc is an essential element which is present in the body at approximately 1 .mu.mole/L. The USDA recommended dietary intake of Zn.sup.2+ is only 15 mg/day, this provides an indication of how little Zn.sup.2+ is required to maintain the required level of this element in a healthy adult..sup.1 Despite this relatively low concentration, Zn.sup.2+ plays an essential role in biology and nutrition. Minor perturbations of normal Zn.sup.2+ levels have been associated with retarded sexual maturation, stunted growth and skin damage. Over 99% of Zn.sup.2+ in biological tissues and fluids is present in a chemically-combined form, with very little present as free Zn.sup.2+ ion. Traditional methods such as atomic absorption effectively measure total Zn.sup.2+ but can not distinguish between the chemically-combined and free forms. The problem of detecting free Zn.sup.2+ is compounded because total free Zn.sup.2+ is decreased only very slightly (50-100 pmol/10.sup.6 cells) in cases of severe Zn.sup.2+ deficiency.
The detection of Zn.sup.2+ in the environment is also important, and is presently an intractable problem ..sup.2 For example, interest in Zn.sup.2+ concentrations in the ocean stems from its dual role as a required nanonutrient and as a potential toxic agent due to its widespread industrial and marine usage. Zinc exists at natural levels in ocean surface water at a total concentration of ca. 0.1 nM..sup.3 Dissolved Zn.sup.2+ concentrations in seawater have been determined using atomic absorption spectrometry, mass spectrometry and voltammetry. The equipment for these procedures is shore based, time consuming and expensive. The concentration data are also inaccurate due to interference from other cations naturally present in sea water. A rapid, selective and more sensitive test for Zn.sup.2+ concentrations which can be performed at sea is desirable.
The current technologies for selectively detecting free Zn.sup.2+ in the presence of the chemically-combined forms have sensitivities in the micromolar range. For example, Zinquin.sup.4 has a rather low sensitivity with association constants approximately 5 orders of magnitude weaker than many metal binding peptides and proteins..sup.5 In addition to its insensitivity, Zinquin cannot be used in the presence of copper.
There remains a need in the art for probes which have excellent sensivity for Zn.sup.2+ and high selectivity for this element in the presence of other transition metal cations.